The present invention relates to paste-form natto and a process for producing paste-form natto.
Natto is one of the Japanese traditional foods. It typically is prepared in the prior art by fermenting whole soybeans with Bacillus natto, and the entire body of the soybean remains even after fermentation. Hereafter, this is called xe2x80x9cprior artxe2x80x9d or xe2x80x9cordinary nattoxe2x80x9d. Natto is increasing its market in Japan every year, because it is a healthy vegetable food, and is reputed to be good for people who are suffering from high blood pressure. Some people do not like its sticky viscous thread and somewhat disagreeable odor.
Industrial scale production of santiary and micro-organism free natto in paste-form is described for use in sandwiches, rice balls (onigiri), Sushi, etc. Prior art technologies for producing paste-form natto include a process for mincing or grinding the natto manufactured by the above prior art method into paste (Japanese Patent Publication No. Hei 9-75025). There also is a process for preparing paste-form natto using roasted soybean flour, called xe2x80x9cKinakoxe2x80x9d, as raw material and then treating a mixture of the flour with water for fermentation (Japanese Patent Publication No. Hei 8-315395 and Japanese Patent Publication No. Hei 11-4662).
However, the process of mincing or grinding the natto produced by the prior art method adds cost to the production and increases the chance of contamination with undesired micro-organisms. Accordingly, mincing the ordinarily manufactured natto into paste cannot be used for commercial production of paste-form natto. Furthermore, the paste-form natto obtained by this process has all of the undesirable properties of the ordinary natto, such as viscous thread, the odor of boiled soybeans, tanned color, bitter taste and uncomfortable rough feeling on the tongue. These properties are not desirable for use in sandwiches, rice balls, xe2x80x9cSushixe2x80x9d, etc.
When xe2x80x9ckinakoxe2x80x9d (roasted soybean flour) is used as a raw material for producing paste-form natto, the natto has disadvantages, such as it takes substantial time to mix the flour with water, and it is very difficult to accomplish the fermentation. Also, aging does not occur uniformly and entirely in the kinako, because the complete culture media is not uniformly fermented, and the natto paste produced from the kinako still has almost all of the undesired properties of the ordinary natto, in addition to its kinako flavor.
The present invention is directed to a novel type of paste-form natto which has a comfortable feeling on the tongue and does not have, or there is a reduction, in the undesirable properties of ordinary natto, including viscous thread, odor, and bitterness. The present invention also presents a process for manufacturing a novel paste-form natto on a commercial scale.
In the present invention, the novel form of natto is made by the process of taking dried soybeans which are dehulled and the kernels sliced to flakes, preferably having a thickness of 0.1-5 mm. The flakes are soaked in water, preferably of 1-2 xc2xd times the weight of the flakes. The wet soybean flakes are steamed, inoculated with a seed culture of Bacillus natto, cultured under aerobic fermentation conditions, and aged under cooling.
When the wet soybean flakes is inoculated with the seed culture of Bacillus natto, lumps of flakes produced during the step of steaming are broken up. Fermentation of the flakes inoculated with Bacillus natto takes place and proceeds to produce the novel paste-form natto.
The paste-form natto thus obtained is packed in containers or bags made of a plastic, such as polyethylene or polyvinyl chloride, and stored in chilled rooms or in refrigerators. The frozen end products of paste-form natto are absolutely or substantially micro-organism free and sanitary. These packed paste-form natto can be supplied to food processing plants without any chance of the natto bacteria leaking out of the product.
Appropriate flavor enhancers or spices may be added to the paste-form natto before packaging and the final product stored in refrigerators or in chilled rooms.
In the present invention, dried soybeans is used as a raw material for manufacturing the novel paste-form natto. The soybeans may be any crop of origin and in any size. Preferably, soybeans of about the same size are collected, and the hulls are removed from the soybean seeds. The soybean kernels that remain after the seed hulls are removed are sliced into pieces, hereafter called flakes, to a thickness of 0.2-5 mm, preferably 0.2-1 mm.
According to the present invention, the sliced flakes are soaked in water at room temperature in an amount of 1-2xc2xd times the weight of the flakes for about 5 to 20 minutes, preferably 5 to 10 minutes. This amount of water as used in the present invention is much less than that used in the process of producing ordinary natto in the prior art, and the time for soaking soybeans is also much shorter than that in the process of producing ordinary natto.
The flakes are spread over the bottom of a vessel, which preferably is flat bottomed, and are steamed under a normal or an increased pressure. Steaming is usually carried out at 105xc2x0-110xc2x0 C. for about 10-20 minutes under a pressure of 0.3-1.5 kg/cm2. Overheating of the wet flakes at this steaming step should be avoided, because succeeding fermentation and aging process will not proceed uniformly and completely.
Normally, the steamed soybean flakes are in a form of lumps like tofu (soybean curd). The lumps are broken up, such as by kneading or applying pressure, to sizes of smaller than about 4 meshes, which correspond to one-fourth of almond grains before or during inoculation with a seed culture of Bacillus natto. The step of breaking up the lumps is hereafter referred to as decomposing the lumps.
The steamed flakes are inoculated with a seed culture of Bacillus natto, such as by spreading the culture on the bottom of the container. The inoculated flakes are then subjected to fermentation. xe2x80x9cInoculationxe2x80x9d means to seed the culture media by sowing (spreading) the seed culture. A preferred form of the seed culture of Bacillus natto is in liquid form, and contains bacterial cells in a number of more than 100 million cells per ml, and is manufactured and sold by Miyagino Natto Co., Ltd. of Japan.
Other commercially available Bacillus natto may be used. It typically is in granules like sesame. This may be said to be xe2x80x9cpre-culturexe2x80x9d of Bacillus natto. Commercial scale production may start with preparation of the seed culture (pre-culture) of the micro-organism in a small scale culture media. According to the present invention, the amount of the seed culture used for the inoculation can be reduced to one-half of the amount normally used in a conventional natto manufacturing process.
If the steamed soybean flakes are not decomposed during or before the inoculation step, incomplete fermentation often may occur, mainly in the center of the batch of the flakes.
The fermentation in the present invention preferably is carried out at a temperature of 40xc2x0-50xc2x0 C. for 15-20 hours under aerobic conditions. Usually, the fermentation finishes in about 17 hours. After the fermentation, the paste-form natto is obtained and is aged at 0-5xc2x0 C. for 12-24 hours in a chilled room. If necessary, or desired, the paste is provided with proper taste enhancers. The product, with or without the taste enhancers, is put into plastic bags or vessels and refrigerated or frozen to form the final product.
The end product, paste-form natto obtained in accordance with the processes of the invention, has a smooth feeling on the tongue, does not have the odor of boiled soybeans and does not have the viscous threads and bitter taste which are common in the ordinary natto. It also has a pale brown color.
Consequently, the natto produced as described can be used in various foods, such as sandwiches, rice balls (onigiri), Sushi, etc., for which often considered unpleasant odor of conventional natto is not preferred. The other recommended uses for foods include milk substitute for children, confectioneries, breads, and hospital foods.